Thanks for the reply. I tried other skin applier demos from the Belleza store, and they all have that neck line thingy. I even tried playing with the Belleza body skin tones, thinking that I probably chose the wrong skin tone, but it seems like I didn't. I don't think it's just my viewer because a really helpful guy, who also helped my husband with his question about his own mesh body, also told me that he can see the line on my neck.
I've asked the Catwa Live Chat support group, and a lot of people suggested different things which I have all tried, including changing the windlight to Calwls, changing my Belleza body neck size to 24 and my Catwa head's neck size to 24, and decreasing my shape's body fat to 24, and even decreasing both the body and head's glossiness. Nothing worked!
I was told that the neck sheath for Belleza is only for system heads. That probably explains why it's bigger than my head's upper neck part.